Rotational Energy Barriers and Relaxation Times of the Organic Cation in Cubic Methylammonium Lead/Tin Halide Perovskites from First Principles

Organic cation dynamics in organic-inorganic hybrid perovskite such as CH3NH3PbI3 have been reported to play an important role in the charge carrier lifetime and ferroelectricity but have not been fully investigated by theoretical approach. We have compared the rotational energy barriers and relaxation times of the methylammonium cation (CH3NH3, MA) in the cubic phases of MABX3 (B = Pb or Sn, X = Cl, Br, or I) by considering full relaxation of the PbX6 or SnX6 inorganic framework from first principles. We successfully reproduced the experimental rotational barrier for the 4 -fold rotational symmetry (C-4) of the C-N axis of MA. Our calculations suggest that the MA can rotate relatively freely because the flexible inorganic framework exhibits liquid-like behavior, which induces cooperative displacement of the metal cation (B) and the halogen (X) via a hydrogen bond and lowers the rotational barrier height. We also demonstrate that the rotational barrier of MA (6-11 kJ mol(-1)), which is closely correlated to the hardness of the PbX6 or SnX6 inorganic framework, can be controlled by choosing the constituent metal cations and halogens. In particular, the rotation barrier heights increase with lighter halogen atoms, which is an important insight for the design of ferroelectric materials.